Superhydrophobic coating

ABSTRACT

This invention solves the problem of reducing the fluid friction resistance accompanying relative movement of surfaces of a solid and a liquid. A superhydrophobic coating acting as a substrate for a gaseous lubricant of very low viscosity, reducing the fluid skin friction, has a hierarchic fracal structural of the surface wherein the forms of the first hierarchic level ( 2, 3, 9 ) are located at the coating&#39;s substrate, and the forms of each successive hierarchic level ( 22, 33, 99 ) are located on the surface of the previous hierarchic level and the forms of individual higher hierarchic levels reiterate the forms of the lower hierarchic levels. Forms of at least two hierarchic levels of rows ( 2, 22 ) and ridges ( 3, 33, 99 ) occur in the coating and, also, the surface has anisotropic geometry, maximally developed fractally in the direction transverse to the direction of flow and maximally smooth in the direction of flow and, also, has channels located in the coating&#39;s substrate to ensure gas flow.

FIELD OF THE INVENTION

The object of the present invention is a superhydrophobic coating usedas a substrate for gaseous lubricant of very low viscosity, reducing thesurface friction of flowing liquids. The invention relates to specificcoatings whose physicochemical properties relative to the liquid incontact with them are referred to as superhydrophobicity. Thisphenomenon, occurring where three phases—solid, liquid and gaseous—arein contact with each other, leads to the formation and retention of anair film on the submerged surface of the coating, which acts as agaseous lubricant of very low viscosity, thus reducing the flow viscousresistance (skin friction). The fields that the invention is related toinclude fluid mechanics, construction of machines involving flowingliquids, water engineering, water transportation, pipelinetransportation of water and other liquids, especially fuels, chemicals,liquefied gases or transportation of water in open channels.

BACKGROUND OF THE INVENTION

The main physical barrier limiting the effectiveness and velocity oftransportation of liquids and in liquids lies in the fact that thedriving systems have to overcome significant resistance accompanying themovement of vehicles or the movement of liquids transported throughpipelines. The aero- and hydrodynamic resistance increases in proportionto the cube of relative velocity of the object and the fluid. One of themany ways proposed to reduce flow resistance of liquids (skin drag) isintentional modification of physicochemical and geometric properties ofsurfaces in contact with a flowing liquid. The relevant solutions tothis problem described so far in the literature can be classified intotwo main categories, referred to as passive: or active ones.

Standard passive methods of lowering the resistance by limiting thedevelopment of turbulence involve, among other methods, modification ofthe geometry of objects around which a liquid flows. Most often flat andsmooth surfaces, once considered optimal from the point of view ofhydrodynanics, are covered with three-dimensional relieves. Known formsof surface three-dimensional arrangements and relieves have the form ofparallel grooves (U.S. Pat. No. 5,171,623), microsteps (U.S. Pat. No.5,133,519), depressions (U.S. Pat. No. 5,171,623) and scales (U.S. Pat.No. 5,114,099) with submilimeter to centimetre dimensions.

Also known are active systems of complex spatially arranged coatingsequipped with a system of mobile, independent projections driven bymicro servo-actuators, or tabs equipped with individual pressure sensorscontrolling each projection separately. Such systems are designed toactively dump turbulence or, to be more precise, to reduce it. Alsoknown are designs with injection of liquid polymers or fine-grainsuspensions, e.g. asbestos or polymer fibres into the inter-face, intothe zone where the vessel's bottom and sides are in contact with waterflowing around them.

In U.S. Pat. No. 5,445,095, a combination of the above-presentedtechnologies has been proposed: injection of polymers onto a surfacewith parallel grooves; similar combinations of active and passivemethods to reduce resistance are also shown in U.S. Pat. No. 4,932,612.

Also a design referred to as “sliding wall” was proposed involving theuse of coatings stationary relative to the liquid and mobile relative tothe flowing body.

The most recent and most effective among the known methods of reducinghydrodynamic resistance involve the reduction or almost completeelimination of immediate contact between the liquid and the object itflows around. It can be achieved by separating the two phases by agaseous phase, for instance, by injecting compressed air into systems ofstraps, channels or shelves placed in the vessel's plating, by injectinga cloud of gas bubbles into the contact zone, or by employing thephenomenon of supercavitation. The injection of gas bubbles into theliquid-solid contact zone can be done through a porous, permeablesurface or through a system of minute channels and nozzles, which inboth cases require the use of gas distribution systems and also, in thecase of submarine vessels, some storage facilities (patent applicationWO 8807956).

It was also proposed to force exhaust gases under the vessel's bottom orto employ the air entrained owing to viscous friction and laterspontaneously sucked in by a system of appropriate nozzles (U.S. Pat.No. 5,545,063).

Some other radical solution reducing hydrodynamic resistance employs theso-called supercavitation a phenomenon involving the formation of aspindle-shaped cavity filled with water steam, moving along with theobject. Such a cavity forms spontaneously around a body travellingthrough liquid environment at velocities greater than 50 metres persecond and requires an appropriate, rounded bow. Such great velocitiesrequire very effective drive systems, e.g. a rocket engine.

A permanent, stable gaseous film (passive “lubricant”) can be mucheasier produced between a solid and liquid (in relative movement to eachother) by making the solid's surface superhydrophobic.Superhydrphobicity means that a liquid has a contact angle between 160and 180 degrees on the surface; in the case of nonwettability by otherliquids, e.g. fats or mineral oils such phenomenon is, calledlipophobicity. An example of material that is both hydro- and lipophobicis fluorine polymer Teflon®; a substance with properties opposite tothose of diamond—a highly hydrophobic material that is wetted byfats—i.e. lipophylic. The phenomenon of superhydrophobicity isconditioned upon appropriate surface geometry. The contact angle ofwater measured relative to a completely flat surface of a solid neverexceeds 140 degrees, even for most hydrophobic surfaces.

Also available on the market are numerous agents for making hydrophobiccoatings, in the form of solutions to be applied onto surfaces, paints,self-adhesive foils, etc. The paints and coatings of this kind have aproperty permitting them to be self-cleaned from particles sticking tothem—they also do not become frosted or iced. Due to the extremely lowhydrophobicity of such artificial coatings a gaseous film is retained onthe surface of a body placed in the bulk of a liquid, with no contactwith its surface. This property permitting a gaseous film to remainunder water has been employed to produce hydrophobic coatings designedto cover e.g., the bottom and the sides of a vessel or the interior ofpipelines, thus reducing the viscous drag (U.S. Pat. No. 5,476,056).

Thus produced decrease in drag relates only to selected parameters ofmovement, and only to the vessel's flat bottom limited by side stripes.The greatest reduction occurs at low velocities: it must be accompaniedby injection of additional portions of gas to make up the losses withinthe irregular gaseous film carried away by the moving, rough surface ofthe liquid flowing relative to the surface and also “scraped off” by theturbulence.

The hydrophobic surfaces employed so far do not have fully controlledgeometry and are usually isotropic. They are formed, for instance, bywax crystallizing on a surface in the form of three-dimensionalcrystallites, or by spraying paints containing chemically hydrophobisedsilica grains.

U.S. Pat. No. 5,476,056 proposes such coatings of controlled geometry tobe formed by e.g., by lithography, screen printing or electroforming. Asa result of employing a chaotic or regular, yet isotropic geometry ofthe relief (patent application WO 0050232), such coatings can reduce theviscous flow resistance to a much lesser degree than it is theoreticallypossible when using a gaseous lubricant. The viscosity of air ishundreds times lower than that of water, and the viscosity of hydrogenis still lower, which is why the viscous drag of the fluid in contactwith a gas film can be theoretically lowered by a factor of over ahundred as compared to a fully wetted immersed surface. Due to theisotropic, and usually chaotic and haphazard geometry of the coatingsused so far, the surface in contact with the liquid's meniscus (theoutermost, monomolecular meniscus i.e. liquid surface film, flexible dueto surface tension) is covered with microcorrugations transverse to thedirection of movement and reproduces, like a negative casting thespatial arrangement of the surface. Consequently, these roughness movingrelative to the passing liquid “scratch” its deeper layers, more distantfrom the coating's surface. This causes the near-the-surface layer to besheared and carried away with the movement of the rigid coating, thelayer being carried away relative to the stationary bulk of the liquid,which leads to internal shearing and favours the creation of turbulence.Conversely, irregularities of the liquid's surface convex in thedirection of the coating, carry away, scrape off and destroy the gaseousfilm.

The process of losing the hydrophobic lubricant is accelerated by “wind”waves, which appear on the liquid's surface—delicate after expansion ofthe gaseous cushion. Those “wind” waves develop due to fast jets of gascarried away with the gas layer. Irregularities of the gas film,thickened by injection of extra air, produce a similar effect. In thepresented system with intense aeration through a single gap in the frontpart of the vessel plating, the coating can only be applied to the flatbottom rather than to the sides. Some parts of such chaotic coating are,due to accidental pattern defects, too irregular for the phenomenon ofsuperhydrophobicity to occur and such thin liquid “bridges” connectingthe liquid and the locally wetted defected surface not only increase thegeneral drag, but can initiate intense turbulence and generate a cloudof bubbles, thus causing increased destruction of the gas film.

Anisotropic, superhydrophobic surfaces, linear in whole or in somesections, produced for experimental purposes by lithography on thesurface of silicon crystals were also described (Bico J., Marzolin C., &Quere D., 1999, Pearl drops. Europhys. Lett. 47(2), pp. 220-226). Therelieves under study contained only a simple system of parallelmicrogrooves and microribbs and the contact angle produced was slightlyin excess of 130 degrees and depended strongly on the direction of themeasurement. This type of surface arrangement has not been-proposed sofar to reduce hydrodynamic resistance.

U.S. Pat. No. 5,054,412 discloses a solution which combines some of theknown technologies described above: a system of macroscopic groovesparallel to the direction of flow, covered with hydrophobic coating. Thegrooves, constituting a structure that counteracts the development ofturbulence, play the role of traps which retain the gaseous film andprotect it against being washed out. The film is produced by injectinggas through a system of nozzles. In this solution macroscopic grooves ofthe same structural level were used

Another system in use is the technology of the underwater flockedcoatings “Sealcoat” manufactured by the Creative Coating Corporation.“Sealcoat” consists of a layer of densely packed short, thin polymerthreads, applied electrostatically directly onto the vessel's bottom orsides covered by appropriate resin. Such a coating, looking like velvetor seal, has high mechanical resistance and is also resistant, in spiteof nontoxicity, to being inhabited by incrustating water organisms. Thuscreated coating is not planned to be hydrophobised. The fibrous“Sealcoat” coating is isotropic and chaotic.

Fibrous textile hydrophobic materials are known as chaotic structuressuch as unwoven and woven fabrics. All known coating structures,designed mainly for the textile industry, are not adapted or designed toreduce hydraulic resistance.

The technological objective of this invention is to give to the coatingsuch geometry that would make it hydrophobic, which means that thewetting angle relative to water or other liquids will be close to astraight angle that is its affinity to gaseous phase will exceed theaffinity to liquid phase.

The superhydrophobic coating, optimized in terms of its ability toreduce viscous flow resistance, should be applied not only under theship's bottom but also on its sides or inside a pipeline. The mainparameters determining usefulness of such coating should be itsmechanical and physicochemical durability, stability and uniformthickness of the gaseous film and the degree to which it reduces viscousresistance as compared to a fully wetted flat surface without anyrelief.

DISCLOSURE OF THE INVENTION

The technological objective as specified above was solved by designing anew coating in accordance with this invention, having a reproducible,three-dimensional anisotropic geometry and structure.

To produce considerable reduction in resistance, the new coating must begiven a precisely defined anisotropic, fractal geometry, in particularone that would depend on the planned direction of fluid flow. Theanisotropy relates to the geometry—the directional character of fractalroughness and to the difference in wettability along differentdirections.

According to this invention, it is proposed to use fully anisotropiccoatings as regards their geometry, that is ones that are linearly andfractally arranged in a scale ranging from tens of micrometers down toseveral manometers, transversely to the direction of movement, and, inextreme cases, of almost molecular size, and smooth in the direction offlow. The fractal dimension of such linear structures results from theirhierarchical structure: from the forms of the first level having theform of grooves and ridges with widths of several to tens ofmicrometers, covered, in turn, by grooves and ridges ranging fromhundreds to thousands nanometers, to the finest ones whose cross sectionhas the size of several nanometers. Considering that only the outermostportions of individual hierarchic levels are wetted, such structuresshould be characterised by a very small surface of effective contactbetween the solid and liquid, even below 1% of the coating surface. Thepresent solution produces a dramatic reduction in drag as compared tosuperhydrophobic coatings known so far, whether chaotic or isotropic.The gaseous film, acting as lubricant in the solution according to thisinvention is thin and has uniform thickness in the direction of themovement, which prevents the formation of wind waves, and the liquid'ssurface is smooth in the direction of flow, and tense, which resultsfrom the fact that it is divided into elongated segments, convex in thedirection of the coating, with a very small curvature radius, fromseveral micrometers to nanometers. Such smooth ridges, stiffened by thehigh surface tension of water, do not permit the liquid meniscus tableto be deformed or hydrodynamic instabilities to be created.

The essence of the solution according to this invention is that thecoating has the surface with a fractal structure of hierarchic structurewherein the forms of the first hierarchic level are located next to thecoating substrate and the forms of each successive level are located onthe surface of forms of the previous hierarchic level and the shape offorms of higher hierarchic levels reiterate the shapes of lowerhierarchical levels and the structure contains forms of at least twohierarchical grooves and ridges; also, the surface has anisotropicgeometry, maximally developed in the direction transverse to the flowand with two gas passages located in the coating's substrate.

The solution according to this invention can have many variants andcombinations that are specified below:

The coating can have a monolithic structure wherein fractal grooves andridges are located directly on the surface of the material's layer:

The coating can have a porous substrate with interconnected pores.

The coating can have a uniform substrate with interconnected channels.

The coating can have grooves and ridges determining an omega-shapedcontour of the coating's cross section.

The coating can have grooves and ridges determining a sinusoidal contourof the coating's cross section.

The coating can have grooves and ridges determining a steplike contourof the coating's cross section

The coating can have a semi-openwork structure wherein fractal groovesand ridges are located in fibres supported by the coating's substrate.

The coating can have a semi-openwork structure wherein fractal groovesand ridges are located in bundles of fibres supported by the coating'ssubstrate.

The coating can have fibres linearly supported by the substrate.

The coating can have an openwork structure wherein fractal grooves andridges are located in hairs attached by their ends in coating'ssubstrate.

The coating can have hairs attached by both ends in the coating'ssubstrate, thus determining loops.

The coating can have hairs with flexible inserts.

The coating can have hairs with a woven layer structure.

The coating can have a woven layer with parallel weave.

The principal advantage resulting from the use of the invention lies ina dramatic reduction of the fluid friction resistance compared torelevant systems known and applied so far.

The invention can be applied in all those situations where a solid andliquid surface move relative to each other and where it is vital toreduce the resistance of that movement to increase the speed and/orreduce the energy expenditure. Furthermore, the superhydrophobiccoatings described above prevent cavitation and the formation ofbuild-ups, encrustations or pits on bodies submerged in water, whichshould extend their lifetime.

BRIEF DESCRIPTION OF THE DRAWINGS

The solution in accordance with this invention is explained in exemplaryembodiments presented in the enclosed drawings, where individual figurespresent the coatings to drawn to scale and with approximate dimensions,with a side length of about 100 micrometers:

FIG. 1—A section of a fractal, linear anisotropic monolithic coating inexemplary embodiment with an omega-shaped cross section, with a solidsubstrate with microchannels and an air gap between the bottom layer andthe substrate,

FIG. 2—A section of a fractal, linear anisotropic monolithic coating inexemplary embodiment with an omega-shaped cross section, with solidsubstrate with micropores and an air gap between the bottom layer andthe substrate,

FIG. 3—Enlarged detail A presented in FIGS. 1 and 2, showing anindividual ridge constituting an element of the coating surface,composed of a number of individual grooves and ridges with sub-microncross-section, which reiterates in a smaller scale the same hierarchicpattern,

FIG. 4—A section of a fractal, linear anisotropic semi-openwork coatingin exemplary embodiment with a solid substrate with microchannels and anair gap between the bottom layer and the substrate,

FIG. 5—A section of a fractal, linear anisotropic semi-openwork coatingin exemplary embodiment with a solid substrate with micropores and anair gap between the bottom layer and the substrate,

FIG. 6—Enlarged detail of a single element B1 determining the outsidesurface of the fractal variant of a semi-openwork or openwork coatingwith a star-shaped cross section, reiterating the same hierarchicpattern in a smaller scale,

FIG. 7—Enlarged detail of a single element B2 determining the outsidesurface of the fractal variant of a semi-openwork or openwork coatingcomposed of numerous elementary fibres with a star-shaped cross sectionwith sub-micron “rays” bound to a cylindrical core, reiterating the samehierarchic pattern in a smaller scale,

FIG. 8—Enlarged detail of the fractal, linear anisotropic coating with ahybrid, lamellar, sandwich-type, microchannel substrate, composed of twomaterials, one forming grooves and the other one ridges, supported bycolumns, and having an air gap between the between the bottom layer andthe substrate,

FIG. 9—Enlarged detail C as shown in FIG. 8, showing an individual ridgewith a rectangular cross section, determining the surface of the linearprojection on the coating, composed of elementary ridges, reiteratingthe same hierarchic pattern in a smaller scale,

FIG. 10—A section of a fractal, linear, anisotropic, openwork coating inthe exemplary embodiment with parallel fibres or bundles of bound fibresconnected by means of pointwise-mounted supports with the solidsubstrate with microchannels, having an air gap between the between thebottom layer and the substrate,

FIG. 11—A section of a fractal, linear, anisotropic, openwork coating inthe exemplary embodiment with parallel fibres or bundles of bound fibresconnected by means of pointwise spacers/support elements with the solidsubstrate with micropores, having an air gap between the between thebottom layer and the substrate,

FIG. 12—A section of a fractal, linear, anisotropic, openwork coatingwith parallel fibres with a star-shaped cross section or bundles ofbound fibres connected by means of continuous spacers/support elementswith the solid substrate, having an air gap between the between thebottom layer and the substrate,

FIG. 13—A section of a fractal, linear, anisotropic, openwork coatingwith parallel fibres with a star-shaped cross section or bundles ofbound fibres connected by means of continuous transverse spacers/supportelements with the solid substrate with micropores, having an air gapbetween the between the bottom layer and the substrate,

FIG. 14—A section of a fractal, linear, anisotropic, openwork coatingwith an outside cover of hairs composed of parallel fibres with astar-shaped cross section or bundles of bound fibres connected in apointwise fashion with the solid substrate with microchannels, having anair gap between the between the bottom layer and the substrate,

FIG. 15—A section of a fractal, linear, anisotropic, openwork coatingwith an outside cover of hairs composed of parallel fibres with astar-shaped cross section or bundles of bound fibres connected in apointwise fashion with the microporous substrate, having an air gapbetween the between the bottom layer and the substrate,

FIG. 16—An individual hair made of a simple fibre or a bundle of fibrescomposed in a fractal fashion, bound pointwise with the coating'ssubstrate,

FIG. 17—An individual hair of another variant, flexible in the planeparallel to the coating, always orienting parallel to a local streamline, bound pointwise to the coating's substrate,

FIG. 18—An individual hair in perspective view I-I as marked in FIG. 17,

FIG. 19—An individual hair in perspective view II-II as marked in FIG.17,

FIG. 20—A section of a fractal, linear, anisotropic, openwork coatingwith an outside cover composed of loops of hairs, with a substrate withmicrochannels, having an air gap between the between the bottom layerand the substrate,

FIG. 21—A section of a fractal, linear, anisotropic, openwork coatingwith an outside cover composed of loops of hairs, with a substrate withmicropores, having an air gap between the between the bottom layer andthe substrate,

FIG. 22—A section of a fractal, linear, anisotropic, openwork coatingwith an outside cover of hairs in the form of woven loops, with asubstrate with microchannels, having an air gap between the between thebottom layer and the substrate,

FIG. 23—A section of a fractal, linear, anisotropic, openwork coatingwith an outside cover of hairs in the form of woven loops, with asubstrate with micropores, having an air gap between the between thebottom layer and the substrate,

FIG. 24—Types of Jacquard weaves,

FIG. 25—A block diagram showing complex morphology and dynamics of thewater table (surface) under the gaseous cushion,

FIG. 26—a block diagram presenting the morphology of the steady,anisotropic and static in time morphology of the water table (surface)over the gaseous film present over the anisotropic linear monolithicsurface made in accordance with his invention.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The coating in accordance with the present invention can be made in amonolithic, semi-openwork, and openwork variants.

Monolithic coatings are entirely made of solid or porous, yet bound intoone lump of material. On the material's outside surface 1, there arefractal grooves 2 and ridges 3, whose cross section determines andomega-shaped line (as in the example shown in FIGS. 1 through 5), aregular zigzag, a high-amplitude sinusoid or rectangular bars (as shownin the example in FIG. 8). Facades of their first-level forms, spaced atdistances not greater than 30 micrometers, are covered with grooves 22and ridges 33 with cross sections of similar shape but of much smallersize, ranging from hundreds to tens of nanometers, and the latter arecovered with another row of the finest, linear forms with a size ofseveral nanometers, which are not shown in the figure.

The material's layer 1 is connected through channels 4 with gap 5located under the substrate 6.

Monolithic structures are best suited to be applied in environmentswhere they are particularly susceptible to mechanical damage. Coatingsmost appropriate in such conditions are those capable of self-repair.Such monolithic coatings made in accordance with this invention will becomposed of stacks of thin layers of materials (laminates) orientedperpendicular to the coating's surface. A stack with fractal,hierarchical pattern of changes in thickness, composed, for instance,alternately of layers resistant and less resistant to abrasion or havingvarious solubility in selected solvents different substances, as shownin FIG. 8. It is composed, as can be seen in the cross-section, of ahierarchical system of rectangular columns 99 of the second level,reiterating the same shape in a smaller scale. The repair involvesgrinding and polishing of damaged places and subsequent grinding by sandblasting or the action of ultrasonic waves after covering the surfacewith fine grained abrasive suspension, or by etching with an appropriatechemical reagent. As a result of selective abrasion the surfaces reliefcan be spontaneously precisely reproduced down to details of nanometersize.

The semi-openwork coatings forming bands of parallel separate fibreswith a star-shaped cross-section 7, or bundles 8 of fibres 7 connectedwith a core, in a fractal version, reiterating the same hierarchicpattern, which are bound with material's layer 1 over the entire length(FIGS. 3, 5).

In embodiments shown in FIGS. 10, 11, 12 and 13 the layer is composed ofbands of individual fibres 7 with a star-shaped cross section, orbundles 8 of fibres connected with a core, bound with the material'slayer 1 by pointwise supports 10 or continuous transverse supports 11.

The most effective coatings as regards drag reduction are openwork ones.Such coatings are covered with hairs 12 parallel to each other and tothe substrate. The hairs have the form of individual fibres 7 with astar-shaped cross section or composed of fractal bundles 8 of boundfibres of the same type with a round core, with a width to length ratio,preferably thicker at the base where they are bound to the material'slayer 1, and perpendicular to it over a small section (FIGS. 14 and 15).

Such an element, resembling a flexible skid, deforms the water table inthe least degree, transversely to the direction of flow. It is mostadvantageous from the hydrodynamic point of view, although mostdifficult technologically, to make hairs 12′ become thinner over thelength, with surface arrangement in the form uniformly spaced groovesand ridges with, invariably parallel to each other in planesperpendicular to the substrate in spite of decreasing hair diameter 12′.(FIG. 19). Such geometry prevents stream lines from being “squashed”,prevents blocking the flow and development of turbulence of the liquid,which would develop in the liquid entering channels as they tend tonarrow towards the end of a hair 12. Hairs 12, 12′, attached by one endto the substrate, e.g., by means of electrostatic flocking—can—bedensely scattered, at distances of ten to twenty micrometers, but theyshould not contact with each other.

In particular, it is possible to use this method to make coatings adaptthe direction of their anisotropy and, consequently, the direction ofthe least resistance, to the varying flow direction. There is thus amethod of creating versatile coatings that surrender to stream linestransverse to the direction of a water vehicle, to be used underturbulent flow conditions, on stormy/troubled turbulent waters inmountain rivers. Such a result can be produced by drastically reducingthe stiffness of the bend above the base of each hair 12, 12′. it isadvisable to make that section of the fibre) as a geometricallycontinuous, although having a non-uniform internal rheologicalstructure, composed of a number of thin, flexible inserts 13 made offlexible elastomer, interspaced with sections of stiffer material ofwhich the rest of the hair 12, 12′ is made. The very segmentationensures full flexibility to the joint, but only in a definite anglerange (FIG. 16). Having such a structure, each hair 12, 12′ will orientthemselves in conformity with a local stream line. The hairs 12, 12.1′can also be attached to the substrate by their both ends, in the form ofpre-stressed loops (FIGS. 20 and 21), but such a solution, like themonolithic or semi-openwork version, is suited only to one direction offlow.

This embodiment can also have the form of a woven layer 15 (FIGS. 22 and23). In contrast to regular Jacquard weaves composed of bands of fibresor threads wavy or twisted in a haphazard fashion, the woven layer 15 asproposed here will be made of separate, elementary or composite fibres(FIG. 24) piercing the substrate: a thick woven fabric, unwoven fabricor foil having a system of parallel weaves. A number of neighbouringelementary weaves can form transverse ridges which produce in the watertable corresponding “negative” grooves transverse to the direction ofmovement and increasing the drag. That is why it is advantageous to makesuch a weave with a haphazard or regular shift of said elementary,identical parallel loops, as in satin, twill, or herringbone weavevariants.

Two basic versions of the material's layer 1 are planned as possiblealternatives to be used in all the monolithic, semi-openwork, andopenwork systems described above.

In the first embodiment it is planned to equip the substrate plate witha system of channels 4, drilled in regular intervals or spaced in achaotic manner (FIGS. 1, 4, 8, 10, 12, 14, 20, 22).

In the second embodiment, the layer 1 has the form of a plate made ofporous material with interconnected pores of micrometer size and withbare or additionally covered with a hydrophobic membrane, impermeable toliquids and easily permeable to gases, e.g., GoreTex® type membranes(FIGS. 2, 5, 11, 13, 15, 21, 23).

Another, thinner plate 6 is attached to layer 1 on the material's sideby means of spacers/connectors: pins, columns, rivets, etc. The spacebetween the two plates forms a gap 5 for the system distributingadditional gas which, in some situations, is necessary for the properfunctioning of the immersed superhydrophobic coating, although basicallyunder steady-state conditions the gaseous film can form and be retainedspontaneously by degassing and evaporating the liquid surrounding anextremely hydrophobic coating. The gas supplied to the coating's surfacethrough a system of gaps 5 and channels 4 (compressed air or hydrogenhaving very low viscosity), which is needed in some extremeapplications, balances the hydrostatic forces exerted by the liquidsurrounding the immersed body. The pressure, which increases with depth,compresses and thus makes thinner the gaseous film. The supplied gascompensates for this decrease in volume and also makes up the losses ofthe gaseous film, which is dispersed and dissolved in the surroundingliquid, as well as scratched off during movement.

Proper selection of material for the coating affects markedly itsdurability and depends on the planned working environment. For pipelinestransporting clean water, free of suspensions, the material's mechanicalproperties are not important; what matters is only its resistance tocorrosion and that is why polymers are the optimal solution in thiscase. For surface and submarine vessels more suited are ceramicmaterials, glass, stainless steel, alloys resistant to seawater,titanium and selected polymers. In the case of small floating vesselssuch as canoes, surfing boards, or small yachts the surfaces resistanceto impact, rubbing against the bottom and other floating objects andanimals, gravel and sand on the beach, and abrasion due to loosematerial suspended in the water call for coating material of extrememechanical properties, such as diamond, diamond-like graphite, silicacarbide, corundum, composite materials and elastic materials with highabrasion resistance, such as some elastomers. It is also possible, aspreviously mentioned, to select the material and design the innerlamellar structure of the coating so as to give it the ability toself-reproduce, in spite of the abrasion of surface layers.

The selection of optimal agent to make the coating surface hydrophobicdepends on many factors, yet primarily on the working environment andphysicochemical compatibility (affinity) with the material of which agiven coating is made. In the case of pipelines transporting clean waterthe entire coating can be made, for instance, of Teflon®, a highlyhydrophobic, soft fluorine polymer, or its derivatives. Similarlydiamond or silicon carbide are substances with natural hydrophobicity.If other materials are used as substrate for the coating we can choosefrom among numerous inorganic substances, primarily silanes orfluorinated polymers, such as trifluoromethylene and its derivatives,soluble in water or organic solvents, which, after evaporation of thelatter, form a thin uniform membrane or one can use fluorinatedchemically or thermally set resins. Some of their formulas are used towater-repellent various kinds of glass, ceramic materials or tomaterials of mineral origin—other substances are applied to polymers andmetallic surfaces. Synthetic and natural waxes, whose earliestapplication was to waterproof fabrics to make them water resistant, arevery soft substances, susceptible to abrasion, yet in some cases theycan be applied to hydrophobise elements less exposed to mechanical wear.

One should be prepared for degradation of the hydrophobic chemical layerapplied to the surface during its lengthy operation; it can happen byabrasion, chemical or photochemical decomposition; the surface can alsobe covered with dirt, oil derivatives, incrustating water organisms anddeposits, in spite of the fact that a superhydrophobic surface has aninherent capacity for self-cleaning. In such a case the surface must becarefully cleaned to remove old layers of hydrophobic agent anddeposits; it can be accomplished by washing with a high-pressure jet ofsolvent or a solution of surfactant and then uniformly re-coated with anappropriate substance.

The block diagram (FIG. 25) shows a complex morphology and dynamics ofwater surface (table) under a gas cushion. In conventional technologies,the cushion is produced by forcing air in between the fractalsuperhydrophobic coating (for the sake of clarity, made transparent inthe drawing) and the liquid surrounding the liquid.

Most important components are represented by the following symbols:

Q—a developing gas bubble, R—a gas bubble totally surrounded by theliquid, S—a turbulent vortex developing in water, T—a turbulent vortexdeveloping in the gas, U—the direction of relative movement of thecoating relative to the surrounding liquid, V—the air flowing in throughthe slot (located outside the area shown in the picture), W—the surfaceof a liquid bridge connecting the defective or locally damaged surfacewith the surrounding liquid.

The block diagram (FIG. 26) shows a steady, anisotropic,static-over-time morphology of the water surface (table) under thegaseous film maintained over the anisotropic monolithic linear coatingmade in accordance with this invention. A thin film, smooth in thedirection of movement, forms spontaneously as a result of extremeanisotropic hydrophobicity of the fractal coating. The water table isdenoted by M and the gaseous film by P.

1. A superhydrophobic coating used as a substrate for gaseous lubricantof very low viscosity, which reduces the fluid skin friction drag,having a developed three-dimensional surface with concave and convexforms situated parallel to each other and to the fluid flow direction,characterised in that it has a fractal structure of hierarchic design,whose forms of the first hierarchic level (2), (3), (9) are located nextto the coating substrate, whereas the forms of each successivehierarchic level (3), (33), (99) are located on the surface of forms ofprevious hierarchic levels, while the forms of individual structures ofhigher levels reiterate the shape of forms of lower hierarchic levelsand the structure comprises forms of at least two hierarchic levels ofgrooves (2), (22) and ridges (3), (33), (99) and the surface hasanisotropic geometry, maximally developed fractally in the directiontransverse to the direction of flow and as smooth as possible in thedirection of flow and has channels located in the coating's substrate toensure gas flow.
 2. The coating of claim 1, characterised in that it hasa monolithic structure wherein the fractal grooves (2), (22) and ridges(3), (33), (99) are located directly on the surface of the material'slayer (1).
 3. The coating of claim 2, characterised in that it has aporous substrate with interconnected pores.
 4. The coating of claim 2,characterised in that it has a uniform substrate, equipped withinterconnected channels (4).
 5. A coating of claim 3, characterised inthat the grooves (2), (22) and ridges (3), (33) determine anomega-shaped contour of the cross section.
 6. The coating of claim 3,characterised in that the grooves (2), (22) and ridges (3), (33)determine a sinusoidal contour of the cross section
 7. The coating ofclaim 3, characterised in that that the grooves (2), (22) and ridges(9), (99) determine a steplike contour of the cross section
 8. Thecoating of claim 1, characterised in that it has a semi-openworkstructure wherein fractal grooves (2), (22) and ridges are situated infibres (7) supported by the coating's substrate.
 9. The coating of claim1, characterised in that it has a semi-openwork structure whereinfractal grooves (2),(22) and ridges are situated in bundles (8) offibres (7) supported by the coating's substrate.
 10. The coating ofclaim 8, characterised in that the fibres (7) are supported by thecoating's substrate in a linear fashion.
 11. The coating of claim 8,characterised in that the fibres (7) are supported by the coating'ssubstrate in a pointwise fashion.
 12. The coating of claim 1,characterised in that it has a openwork structure wherein the fractalgrooves (2), (22) and ridges (3), (33) are located in hairs (12), (12′)whose bases are set in the substrate of the coating.
 13. The coating ofclaim 11, characterised in that the hairs (12), (12′) are attached withtheir both ends in the substrate of the coating, thus forming loops. 14.The coating of claim 11 claim 11, characterised in that the hairs haveflexible inserts (13).
 15. The coating of claim 1, characterised in thatthe hairs (12), (12′) have the structure of a woven layer (15).
 16. Thecoating of claim 13, characterised in that the woven layer (15) has aparallel weave.